1. Field of the Invention
The present invention generally relates to communications and, more particularly, to a method and a multimode terminal for minimizing a mute interval caused when call interruption occurs due to handover between communication networks which provide communication services by means of different mobile communication technologies.
2. Description of the Related Art
Currently, a mobile communication network is being developed from a 2G network, i.e. a Code Division Multiple Access (CDMA) network and a Global System for Mobile communication (GSM) network, to a 3G network, such as a Wideband CDMA (WCDMA) network. Further, since service areas using a 2G network have been widely distributed and have been completely equipped with basic facilities, service areas using a 3G network have increased while transiently using the basic facilities of such a 2G network. Accordingly, in such transient situations, a terminal designed to receive the specified service of the 3G network has been manufactured as a multimode terminal which also supports the existing 2G network.
Such a multimode terminal supports both an idle handover function, through which a WCDMA mode can be switched to a CDMA mode in an idle state, and vice versa, and a traffic handover function through which switching to a CDMA mode can be accomplished without communication interruption when the terminal enters a WCDMA shadow area during communication. Such functions are efficient in terms of using the CDMA network (2G network). However, since a handover must be accomplished between different modems within a short time, internal hardware and software of the terminal become complicated.
Since a handover between a WCDMA system and a CDMA system corresponds to a handover between different types of systems in which Radio Access Technologies (RAT) are different, the handover will be referred to as an inter-RAT handover. Further, since the handover corresponds to a handover between different types of systems, it corresponds to a hard handover in which voice interruption occurs when changing the systems, differently from a soft handover with no voice interruption. Therefore, a mute interval inevitably occurs in handover between communication networks which provide communication services by means of different mobile communication technologies. Reducing such a mute interval is an important goal of the inter-RAT handover technology.
FIG. 1 is a block diagram illustrating a terminal which performs a traffic handover according to the prior art.
When the terminal shown in FIG. 1 is located in a WCDMA network, a CDMA RF 220 and a CDMA modem 240 are temporarily powered off. However, since an antenna, a duplexer 210, a WCDMA RF 230 and a WCDMA modem 250 are powered on, the terminal operates in a WCDMA mode. Further, when the terminal is located in a CDMA network, the CDMA RF 220 and the CDMA modem 240 are powered on, but the antenna, the duplexer 210, the WCDMA RF 230 and the WCDMA modem 250 are temporarily powered off. Accordingly, the terminal operates in a CDMA mode.
An operation of the terminal performing a traffic handover to a CDMA network from a WCDMA network will be described with reference to FIG. 2. Generally, if communication begins in a WCDMA mode, a controller 200 turns on the CDMA modem 240, and the CDMA modem 240 goes into a low power mode and waits to receive a message. Then, as the handover occurs, if the WCDMA modem 250 receives a handover from a Universal Mobile Telecommunications System Terrestrial Radio Access Network (UTRAN) command from a WCDMA base station in step 300, the WCDMA modem 250 communicates with the controller 200 and activates the CDMA modem 240. In step 305, the WCDMA modem 250 extracts a Universal Handover Direction Message (UHDM) from the received handover from the UTRAN command message, and transfers the extracted UHDM to the CDMA modem 240. If the UHDM is received from the WCDMA modem 250, the CDMA modem 240 enters a service mode in order to acquire a handover system in step 320, and performs acquisition of the handover system in step 325.
Separately from the operation of the CDMA modem 240 as described above, the WCDMA modem 250 transfers a handover report to the controller 200 in step 310. In step 315, the controller 200 transfers disconnect Dual-Port Random Access Memory signals, which disconnect a DPRAM, to the WCDMA modem 250. In step 330, the controller 200 transfers connect DPRAM signals, which connect the DPRAM, to the CDMA modem 240. In other words, a switching operation is performed, in which a path, through which the WCDMA modem 250 transfers received voice signals to the controller 200 through the DPRAM, is changed to the CDMA modem 240.
In order to perform the switching operation, the voice path of the WCDMA modem 250 is first blocked. From this point, there occurs a mute interval which a user experiences. After the voice path is blocked, the controller 200 releases the DPRAM connection of the WCDMA modem 250, i.e. the switch connection, and initializes the DPRAM connection of the CDMA modem 240. Since this corresponds to a change in a hardware path, when a DPRAM synchronization is made between the controller 200 and the CDMA modem 240 after a predetermined amount of time passes, it is possible to normally exchange data through the DPRAM. However, invalid data may be transmitted in the process of initializing the DPRAM. If the process of initializing the DPRAM connection fails, the handover processing ends in a failure and communication ends.
However, if the process of initializing the DPRAM connection ends successfully, the CDMA modem 240 changes the voice path to the CDMA, and waits to receive voice signals. This process is performed in parallel with a process in which the CDMA modem 240 processes the UHDM. Since the handover processing has not yet ended at this time point, the mute interval continues. Further, even when there occurs a problem in the course of processing the UHDM, the handover processing ends in a failure and communication ends.
Accordingly, only when the UHDM is processed and a Handover Completion Message (HCM) is transmitted in step 345 via steps 332 and 335, a voice path is established in step 340. In step 350, a voice path change is reported and voice signals are actually received through the CDMA modem 240. In step 355, the CDMA modem 240 receives an acknowledgment message for an HCM. In this way, a user can actually be allowed to hear the voice of a communication partner. For example, a time of about 0.9 seconds normally occurs before a user can actually be allowed to hear the voice of a communication partner after a mute interval in a traffic handover. This time is long enough for the user to detect mute.
As described above, since the handover is performed through the switching operation between the modems, the mute interval continues because it is possible to maintain the output of voice communication until the switching operation is completed. Therefore, a user must wait in the mute state until the switching operation is completed for the handover, and communication is connected. The user is therefore greatly inconvenienced.